FCPX will import the footage directly without anything special required. You should decide and set the project properties for what you want before import. If you know for sure you want HD output, set it to that. The other choice is 4K,and deduce it to HD for the final output.
Depending on the horsepower of your Mac you may get stutters in the playback trying to play 5.4K footage. If that is a problem, switch to editing with proxy footage, and switch back to full res on final output.
